This invention relates to recording material having a color former coating layer formed by coating on a surface of a base sheet a coating composition including microcapsules each encapsulating a hydrophobic material containing an electron donating organic chromogenic material and more particularly to an improvement of preventing undesirable smudging and greasing on such recording material. This invention also relates to a method for the production of such recording material.
Among recording materials there are well known pressure-sensitive copying papers and heat-sensitive recording papers which utilize the color developing reaction between electron donating organic chromogenic material (hereinafter referred to as "color former") and electron accepting acidic reactant material (hereinafter referred to as "acceptor"). In pressure-sensitive copying paper at least one of the color former and the acceptor is contained in microcapsules so as to be isolated from the other and they become into contact with each other by rupturing such microcapsules to develop a color. In a most typical type of pressure-sensitive copying paper minute oil droplets in which the color former is dispersed or dissolved are encapsulated and coated onto papers.
Usually the pressure sensitive copying system utilizing the above-mentioned pressure sensitive copying papers consists of three kinds of sheets such as top sheet, middle sheet and bottom sheet, wherein the top sheet is coated on the underside thereof with a composition consisting mainly of pressure rupturable microcapsules each enclosing an oil droplet containing a color former dissolved or dispersed therein, the middle sheet is coated on the upperside thereof with another composition consisting mainly of an acceptor and also is coated on the underside thereof with the composition of microcapsules containing an oil droplet in which a color former is dissolved or dispersed and the bottom sheet is coated on the upperside thereof with the composition of an acceptor.
The most typical method for making oil-containing microcapsules for the above mentioned purpose is to utilize the coacervation technique. For example, according to the disclosure in U.S. Pat. No. 2,800,457 oil-containing microcapsules are made by the following steps:
(1) A mixture of two different hydrophilic colloid sols in which oil droplets are dispersed is prepared. The mixture may be made by forming an aqueous sol of one colloid material, emuslifying the selected oil therein, and mixing the emulsion with an aqueous sol of another colloid material or the two sols may be made and mixed and the oil emulsified therein. The two colloid materials have opposite electric charges and at least one of them is gellable.
(2) Coacervation is caused by dilution and/or by adjusting the pH of the mixture to form and adhere a coacervate around each of the oil droplets.
(3) The coacervate around each of the oil droplets is gelled by cooling; and
(4) The coacervate is further hardened by addition of a hardening agent and if necessary by adjusting the pH to an alkaline zone.
For the manufacture of pressure sensitive copying papers through the utilization of the above coacervation technique coating additives such as pulp powder, casein, starch and hydroxyethylcellulose and dyes for enhancing the whiteness such as fluorescent dyes, organic pigments and methyl violet are added to the coating composition. For preservation various fungicides, antiseptics and mold proofing agents such as sorbic acid, potassium sorbate, pentachlorophenol, chloramine T, Salicyclic acid, methylnaphthoquinone, butyl p-oxybenzoic acid, formaldehyde and glutaraldehyde may also be added to the coating composition. The coating composition is preferably preserved at a temperature below 20.degree. C.
The pressure sensitive copying system particularly finds its usefulness in business form papers having printed forms applied by an offset printing, preferably by a wet offset printing. In case where the middle sheet of a business form having a color former coating on one side and an acceptor coating on the other side is wound into a roll after a form is printed on its one surface by a wet offset printing, undesirable smudges are often produced on the acceptor coating layer over its all area. This is considered due to the fact that microcapsules enclosing a color former are incidentally ruptured by printing pressure and the color former is transferred to the acceptor layer. In addition to this, in many cases the color former microcapsule coating layer of the top or middle sheet is greased or stained with printing ink. It is considered that various agents included in the coating layer such as protein, surface active agents and binders which may be transferred to the blanket surface and/or the printing roll surface would participate in staining the color former microcapsule layer with printing ink.
An attempt has been made to improve the pressure resistance of the microcapsules by adding binders such as polyvinyl alcohol, casein, starch, sodium alginate, carboxymethylcellulose, hydroxymethylcellulose, hydroxyethylcellulose. However, this attempt has not been able to substantially prevent the above-mentioned smudging and greasing.
The principal object of the invention is to provide an improved recording material having a color former microcapsule layer in which smudges and printing ink stains produced during a wet offset printing operation can be minimized.
Other objects and advantages of the invention will become apparent from the following description.